Alkaline desmutting composition for ferrous metals

ABSTRACT

AN AQUEOUS ALKALINE FERRICYANIDE COMPOSITION IS DISCLOSED FOR REMOVING SMUT FORMED ON THE SURFACE OF FERFOUS METALS BY COMMON SURFACE TREATMENTS SUCH AS PICKLING, CLEANING, DESCALING AND ETCHING.

United States fatent O 3,573,984 ALKALINE DESMUTTIN G COMPOSITION FORFERROUS METALS Dilip G. Shah, Wolcott, Conn., assignor to MacDermidIncorporated, Waterbury, Conn. No Drawing. Filed Apr. 17, 1968, Ser. No.721,946 Int. Cl. C23g 1/20 U.S. Cl. 134-2 5 Claims ABSTRACT OF THEDISCLOSURE An aqueous alkaline ferricyanide composition is disclosed forremoving smut formed on the surface of ferrous metals by common surfacetreatments such as pickling, cleaning, descaling and etching.

This invention pertains to a composition for and a method of removingsmut formations produced on the surface of ferrous metals and alloys,which smut results inherently during many common surface-conditioningtreatments such as pickling, descaling, satinizing and the like to whichsuch metals are subjected. The invention is particularly directed tocompositions which, when dissolved in water, are useful in the foregoingmanner and which are alkaline in character rather than acidic asheretofore more commonly used.

Most acid treatments such as pickling, descaling, satin finishing oretching leave a smut on the surface of ferrous metals and alloys. Thecomposition of this smut varies and depends largely on two factors;namely, the nature of the ferrous metal or alloy, and the composition ofthe acid treatment bath. The amount of smut depends mainly on the degreeof attack by the acid on the surface of the metal. The smut may consistof carbon, graphite, carbides, nitrides, sulfides, phosphides, oxides,etc., of the constituent elements. Whatever the composition of the smut,its removal by means of prior desmutting technologies has left room forimprovement in many respects. Prior treatments include the use of acidssuch as hydrochloric or hydrofluoric, or mixtures of these or otheracids. While relatively smut-free surfaces can be obtained using suchacids, the acids attack most ferrous metals and alloys very rapidly, andaccurate control or buffering of the action is difficult. There is,moreover, a tendency of most ferrous surfaces to rust very rapidly afterbeing subjected to acid treatments, in many cases involving only a veryfew minutes. The use of hydrochloric acid causes the surface to bepitted and hydrogen embrittlement is apt to occur. Also hydrogen gas isformed and evolved in the process and constitutes a hazard unless careis taken to see that there is proper ventilation.

Some use has been made of electrolytic cleaning or desmutting of metals,using a highly alkaline electrolyte medium in which the parts to becleaned are immersed, as in a barrel or on racks while being advancedthrough a tank, during which a direct current voltage is impressedbetween the parts and electrodes placed in the solution. The desmuttingaction in this case is accomplished primarily by gas evolution from thesurface of the parts being cleaned, being more in the nature ofmechanical operation and the smut is deposited as an insoluble sludge orcake in the cleaning tank and must be removed periodically.

In another known procedure, sodium or potassium permanganate with sodiumor potassium hydroxide in solution has been used. While this systemeliminates most of the problems encountered with acid baths, the mainobjection to permanganate baths is that an insoluble manganese dioxideis the end product of the reaction. Thus there is generally a film ofmanganese dioxide left on the sur 3,573,984 Patented Apr. 6, 1971 faceof the part, but more importantly the necessary operating conditions oftemperature and immersion time are high, thereby increasing theoperating cost.

It is accordingly a principal objective of the present invention toprovide an improved composition and process for removing smut fromferrous metals and alloys, whereby the problems and disadvantages of theprior technologies are overcome or substantially reduced.

This objective is accomplished by means of an aqueous alkalinecomposition as hereinafter more fully described, in which the articlesare immersed to effect removal of smut from the surfaces thereof. Thenew compositions offer the advantages of lower operating cost byreducing the concentrations necessary as well as the temperature andimmersion time required for satisfactory smut removal. The newcompositions do not attack the base metal, or result in pitting orhydrogen embrittlement in its surface, and do not encounter thepreviously mentioned hydrogen evolution problems. Moreover, no insolublereaction products are formed. When the desmutting composition is spent,since it has negligible toxicity and is nonsludging, there is no wastedisposal problem. Finally, the new compositions have substantial effectin retarding rusting of the clean surface of the metal.

In brief, the new compositions consist of aqueous solutions of aferricyanide ion in alkaline medium. Either sodium or potassiumferricyanide may be used to provide the ferricyanide ions, while thealkaline medium is best provided by sodium, potassium or lithium.hydroxide. The ratio of ferricyanide to hydroxide ion is not critical.The concentration of ferricyanide ion, in order to be effective, shouldbe at least one quarter ounce per gallon of water, and may be anythingup to the limit of solubility of the selected ferricyanide compound inthe resulting solution. The concentration of hydroxide present islikewise not critical but should be sufficient to make the solutiondefinitely alkaline so that the pH is on the order of 13. A particularlyeffective composition comprises about four ounces per gallon of water ofcombined potassium ferricyanide and sodium hydroxide in which the weightratio of the hydroxide to ferricyanide is about one and onehalf to one.

For many types of smut removal, solution temperatures as low as ambientare sufficient, but in that case the immersion time will ordinarily berelatively long. Optimum combinations of temperatures and times toeffectively remove the smut will vary, depending on the metal and typeof acid treatment employed but in general temperatures of F. to boilingand immersion times of five seconds to five minutes cover the bulk ofthe practical applications. In short, operating variables such asconcentration, temperature and time of immersion should be varied withinthe limits indicated, depending on the nature and amount of the smut.

Typical examples are given below for the removal of various types ofsmut produced on several different ferrous articles.

EXAMPLE I A mild steel panel measuring 3 x 4 was immersed in an aqueoussolution at room temperature for 10 min utes containing 25% by volumehydrochloric acid. This produced a heavy smut on the surface of thepanel. The panel was rinsed in cold running water and immersed in adesmutting solution containing four ounces of alkaline potassiumferricyanide per gallon as described above. The desmutting solution inthis case was maintained at F. All smut was removed from the surface ofthe panel within 30 seconds, resulting in a clean, bright surface on thepanel. The panel was rinsed in cold running water and was immediatelyready for further processing such as plating, phosphating, painting,etc, The dried panel when stored for substantial period of time inordinary ambient room temperature and humidity conditions showedremarkably improved characteristics in respect to resistance to rusting.Whereas such a panel does not show rust for up to as much as two weeks,a similar panel employing a conventional acid desmutter will rust in amatter of minutes under identical conditions.

EXAMPLE II Again a mild steel panel the same as that in Example I wasprocessed in an acid solution, this time containing by volume ofsulfuric acid in place of the hydrochloric acid. A heavy dark smut wasproduced on the surface of the panel. The panel was rinsed and subjectedto immersion in the same desmutting solution as described in Example I.A clean, bright and smut-free surface was obtained within seconds asbefore.

EXAMPLE III Another mild steel panel of the same type was processed innitric acid solution instead of hydrochloric. A heavy dark smut wasproduced on the surface. Again the panel was rinsed in water andsubjected to desrnutting in the solution described in Example I. Aclean, bright smutfree surface was obtained within 30 seconds.

EXAMPLE IV A stainless steel, Type 434, panel was etched for 10 minutesat F. in an aqueous solution comprising 10% by volume of hydrochloric,5% by volume of nitric acid and 25 by volume of phosphoric acid. Thepanel was rinsed in cold running water and was found to be completelycovered by a heavy black smut. A desmutting solution was preparedcontaining 1 ounce per gallon of alkaline ferricyanide and maintained ata temperature of 'F. The smutted panel was immersed in this solution for30 seconds and then rinsed in cold running water. A clean, bright andsmut-free surface was obtained.

EXAMPLE V Parts composed of spring steel were pickled for three minutesin an acid mixture of the same composition and under the same conditionsas that given in Example IV above. The parts were rinsed in cold runningwater and then immersed for 1 minute in a solution containing 4 ouncesper gallon of alkaline ferricyanide at F. After rinsing in cold runningwater, the surface of the parts was brownish due to a light ferrichydroxide layer. This was easily removed by a quick dip (15 seconds) ina 25% by volume hydrochloric acid solution at room temperature. Theparts were then rinsed in cold running water and presented a clean,bright and smut-free surface.

EXAMPLE VI Short lengths of cast iron pipe and elbows were pickled in anacid mixture of the same composition as that given in Example IV for 15minutes at room temperature and rinsed in cold running water. Thesurface so obtained was heavily smutted with a black smut. The pieceswere then immersed for 2 minutes at F. in a 4 ounce per gallon solutionof alkaline ferricyanide. Again the pieces were rinsed in cold runningwater and immersed in a 25% by volume hydrochloric acid solution at roomtemperature for 15 seconds to remove the ferric hydroxide layer. Theywere then rinsed in cold running water. The surfaces obtained were cleanand smut-free.

The new desmutting compositions have been found to be especiallyeffective in preventing blushing or hazing of nickel or other metaldeposits plated onto the base ferrous metal, and to ensure good adhesionof such deposits. Whereas the specific examples above used p0- tassiumferricyanide and sodium hydroxide, the anions are relatively unimportantso long as they do not render the compounds insoluble. These particularcompounds are selected because of their availability and cost, butsodium ferricyanide and either potassium or lithium hydroxide will workas well.

What is claimed is:

1. The method of desmutting the surface of a ferrous metal article whichcomprises immersing the article in an aqueous solution which consistsessentially, in addition to water, of a first member selected from thegroup consisting of sodium, potassium and lithium hydroxide, and asecond member of the group consisting of sodium and potassiumferricyanide; said ferricyanide being present in said composition toprovide from about one-quarter ounce per gallon of water to the limit ofits solubility in solution, and said hydroxide being present in amountsufficient to render the solution alkaline, at solution temperaturesranging from ambient room temperature to boiling, and maintaining saidarticles in said composition until the smut has been removed, removingand then rinsing them in water.

2. The method as defined in claim 1, wherein in the aqueous solution thesaid hydroxide is present in amount equal to about one and one-halftimes the weight of the ferricyanide.

3. The method of claim 1 wherein the solution pH is about 13.

4. The method of claim 1 wherein in the aqueous solution theconcentration of the ferricyanide is about 1.6 ounces per gallon andthat of the hydroxide is about 2.4 ounces per gallon.

5. The method as defined in claim 1, wherein the articles are immersedfor periods of from about five seconds to five minutes.

References Cited UNITED STATES PATENTS 2,566,615 9/1951 Keilholtz et a1.25279.lX 3,098,043 7/1963 Wendell 252-156X LEON D. ROSDOL, PrimaryExaminer A. I. RADY, Assistant Examiner US Cl. X.R. 252-795, 156

